PDCD1 and Sepsis: Further research indicated significant upregulation of co-inhibitory receptors, such as PD-1, cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), B and T lymphocyte attenuator (BTLA), T cell immunoglobulin mucin03 (TIM-3), and lymphocyte activation gene-3 (LAG-3), on T cells from the blood of sepsis patients (Boomer et al., 2012).